The invention is directed generally to electrochemical apparatus for the oxidation or consumption of a fuel and the generation of electricity, such as molten salt or solid electrolyte fuel cells.
Although particular embodiments are applicable to conventional cofired or bonded solid electrolyte fuel cell apparatus, the present invention is particularly useful when utilizing non-cofired or nonbonded solid oxide electrolyte fuel cells, preferably planar, that contain a stack of multiple assemblies. Each assembly comprises a solid electrolyte disposed between a cathode and an anode, being bounded by separators which contact the surfaces of the electrodes opposite the electrolyte. A fuel manifold and an air manifold pass gases through or over the assembly elements, with a gasket sealing the anode adjacent to the air manifold and a gasket sealing the cathode adjacent to the fuel manifold to minimize fuel and air mixing in a zone which would decrease cell voltage.
The fuel cell operates by the introduction of air into the cathode and the ionization of oxygen at the cathode/electrolyte surface. The oxygen ion moves across the gas-nonpermeable electrolyte to the anode interface, where it reacts with the fuel flowing into the anode, releasing heat and giving up its electron to the anode. The electron passes through the anode and separator to the next adjacent cathode.
Hydrogen- or hydrocarbon-containing fuels can be used in the electrochemical apparatus of the present invention, such as hydrogen, carbon monoxide, methane, natural gas, (including landfill gas) and reformed hydrocarbon fuels (including diesel and jet fuel). The gas to be supplied to the cathode can be oxygen or an oxygen-containing gas such as air, NO.sub.x, or SO.sub.x.
Cermet electrodes for cofired, or bonded, solid oxide electrochemical fuel cells, preferably tubular in shape, are disclosed in U.S. Pat. No. 4,582,766 to Isenberg et al. (Westinghouse Electric Company). Electronic conductors (metals) form the electrode and are bound to the electrolyte by a ceramic coating which is preferably the same material as the electrolyte. The metal electrode particles are oxidized and then reduced to form a porous metal layer which contacts both the ceramic coating and the metal electrode particles. The problems of ceramic-metal thermal expansion mismatch are not solved, and are indeed increased by the electrolyte/electrode bonded structure. In the bonded structure, the materials which are to comprise the electrodes and electrolyte are cofired, or diffusion bonded to each other to form a unitary structure.
The cofired or bonded solid oxide electrolyte fuel cells have practically no tolerance to sulfur bearing fuels. The performance of the cofired or bonded solid oxide electrolyte fuel cells degrades considerably when used in a process to utilize sulfur bearing fuels, even at concentrations as low as 1 part per million (ppm).
Isenberg et al. in U.S. Pat. No. 4,582,766, and Isenberg in U.S. Pat. No. 4,597,170, propose to use nickel, cobalt and alloys or mixtures thereof as the electrode conductive material, as being more sulfur resistant than other metals or metal oxides.
The performance degradation experienced using sulfur bearing fuels with cofired or bonded solid oxide electrolyte fuel cells prompted Westinghouse to modify the anode bonded to the electrolyte by coating the anode with a gas permeable oxygen-ionic-electronic conductor material coating which was sinter or diffusion attached, disclosed in U.S. Pat. Nos. and 4,702,971 and 4,812,329 to Isenberg. A cell having such a coated fuel electrode was tested for 16 hours using a hydrogen, carbon monoxide, water vapor fuel containing 50 ppm hydrogen sulfide and experienced 4.7% performance loss. Extended operation, or thermal cycling caused the anode coating to crack and flake off, however, and resulted in the poisoning of the underlying bonded anode.
Additional sulfur tolerance test results by Westinghouse for cofired or bonded fuel cells are contained in the final technical report to the U.S. Department of Energy, "Anode Development For Solid Oxide Fuel Cells", Report No. DOE/MC/22046-2371, December 1986. Various anode materials were tested in cofired or bonded fuel cell designs using a hydrogen, carbon monoxide, water vapor fuel containing sulfur species in amounts of 2 ppm, 10 ppm, 25 ppm and 50 ppm. The report concluded that cell performance degraded rapidly for about the first two hours of sulfur bearing fuel utilization, and at a slow, linear rate thereafter, in the presence of as low as 2 ppmv H.sub.2 S in the fuel. The incorporation of cobalt in the bonded anode did not improve sulfur tolerance, while impregnation of the anode with nickel and samarium doped cerium oxide reduced cell degradation (although performance gradually decreased with time, attributed to film cracking and sintering). The degradation in fuel cell performance was found to be reversible, with recovery of cell voltage and resistance when the sulfur was removed from the fuel.
The level of sulfur represented by 50 ppm hydrogen sulfide in hydrogen is not the worst case that would be experienced by the use of potential sources of fuel for fuel cells. Fuel gases derived from liquid fuels such as diesel and jet fuels, coal derived gases and landfill gases may contain hydrogen sulfide levels of about 300 to 1000 ppm, or even higher.
It is an object of the present invention, therefore, to provide a fuel cell capable of utilizing sulfur bearing fuels with stable operating performance over extended periods of time.